Epic Games Store, in an attempt to make you download their client, continues to offer games for free, but this time, if until now you have managed to resist, it will be really difficult not to give in to temptation since for the Free it's nice of day we have World of Goo.
World of Goo is a puzzle game released in 2008 that has won loads of prizes (all deserved). The game consists of creating structures through the use of slime balls, called Goo, and reaching the exit level with a certain minimum number of Goos.
Obviously there are many different types of Goo to use to reach the exit, each with its specific properties.
Among the many awards that the game has had is also the inclusion in the 2010 book of the "1001 Video Games You Must Play Before You Die", In addition to the title of Game of the Year by IGN. Since now the opportunity presents itself for free, you really should try it.
You can find more information on the Epic Games Store page dedicated to the game.